Every project should have a contingency allowance for both time and budget as there is always a tendency to under-estimate tasks, particularly when under pressure to deliver as soon as possible. Indeed, in practise, many project managers are not given the luxury of estimating the total time required but are likely to be given a fixed end date and the project tasks have to be fitted into the time available.
The business requirements document should accurately, and in detail, describe the purpose of the project. It states what is needed to achieve that goal i.e. what is in-scope, what is out-of-scope, any assumptions that have been made, any constraints that have been imposed and expected timescales.